1. Field of the Invention
The present invention relates to a union joint and, more specifically, to a union joint for use in semiconductor industries requiring perfectly sealed connection of pipes.
2. Prior Art
Semiconductor device manufacturing processes for manufacturing semiconductor devices, such as ICs and LSIs, are carried out in clean rooms in which elaborate precautions are employed to reduce dust particles and other contaminants in the air. A special gas for use in the clean room is supplied through piping from an external gas source. The special gas must be extremely pure and the existence of foreign gases, such as air, in the special gas must be perfectly obviated. Therefore, the piping for supplying the special gas must be formed of precision pipes and precision union joints for connecting the pipes. Union joints to be used in combination with high pressure vessels also must be capable of perfect sealing in view of safety.
The perfectly sealed connection of pipes by a conventional union joint may be attained by increasing pressure acting on a metallic gasket provided between the pipes. However, if an excessive torque is applied to the coupling ring to join pipes firmly, the pipes and a gasket interposed between the pipes are distorted torsionally, and hence the torque cannot be increased beyond a fixed limit.
The applicant of the present patent application proposed previously a union joint as shown in FIG. 9 incorporating a thrust bearing to transmit only an axial force from the inner surface of a coupling nut to one of two pipes and not to transmit torque from the inner surface of the coupling nut to the pipe. As shown in FIG. 9, the union joint consists of a first sleeve 10 attached in a sealed joint to a first pipe 1, a second sleeve 20 attached in a sealed joint to a second pipe 2, a metallic gasket 30 interposed between the sealing surface 15 of the first sleeve 10 and the sealing surface 25 of the second sleeve 20, a coupling nut 5 for axially drawing together the first sleeve 10 and the second sleeve 20 to join the first pipe 1 and the second pipe 2 hermetically, and a thrust bearing 8 placed between the inner bottom surface 6 of the coupling nut 5 and the shoulder of the first sleeve 10. When the coupling nut 5 is turned to draw together the first sleeve 10 and the second sleeve 20, hence the first pipe 1 and the second pipe 2, only an axial force acts on the first sleeve 10, so that the pipes 1 and 2 can be firmly joined together by applying a large torque to the coupling nut 5 without torsionally distoring the pipes 1 and 2 and the metallic gasket 30. In order to enhance the sealing effect of the union joint, the sealing surface 15 of the first sleeve 10 and the sealing surface 25 of the second sleeve 20 are mirror-finished and, to enhance the possibility of repetitive use, the sealing surfaces 15 and 25 are burnished. The metallic gasket 30 having excellent compressibility, capable of securing residual elasticity, capable of functioning without contaminating the gas and having excellent durability can be used repeatedly to meet economical requirements.
When the coupling nut 5 is turned to draw together the sleeves 10 and 20, the metallic gasket 30, which is in line contact with the sealing surfaces 15 and 25 of the sleeves 10 and 20 along contact lines CL, is squeezed elastically in a shape indicated by broken lines in FIG. 10 and is brought into surface contact with the sealing surfaces 15 and 25 of the sleeves 10 and 20 along contact areas CA as shown in FIG. 11. Thus, the sleeves 10 and 20 are perfectly hermetically joined together to isolate the inside space Ai enclosed by the sealing surfaces 15 and 25 and the metallic gasket 30 perfectly from the outside space Ao. In FIG. 9, indicated at 20a is an external thread formed on the second sleeve 20, and at 5a is an internal thread formed on the coupling nut 5 to be screwed on the second sleeve 20.
The reliability of this previously proposed union joint is not satisfactory. In some cases, the union joint is unable to prevent leakage when a high-pressure gas of a pressure on the order of 150 kg/cm.sup.2 is passed through the union joint even if the sleeves 10 and 20 are drawn axially toward each other by an increased force and the components are finished in the highest possible accuracy.
Even if such union couplings are manufactured in a lot under quality control, some of the union couplings may be defective and the union joints may differ from each other in quality. Accordingly, causes of leakage and quality variation must be elucidated and techniques for eliminating such disadvantages must be developed.
Still further, if the metallic gasket 30 is located correctly between the respective sealing surfaces 15 and 25 of the sleeves 10 and 20, it is possible that the metallic gasket 30 protrudes partly into the passage between the pipes 1 and 2 to reduce the flow passage area. Accordingly, it is desired to develop a technique to locate the metallic gasket 30 properly between the respective sealing surfaces 15 and 25 of the sleeves 10 and 20.